Posadas became the leader of the Latin America Bureau of the Fourth International and, under his guidance, the movement gained some influence in the region, particularly among Cuban railway workers, Bolivian tin miners and agricultural workers in Brazil.

The "Posadists" founded their own Fourth International in 1962, which started using the name Fourth International (Posadist) only at a later time (in the early 1970s). At their founding conference the movement proclaimed that “Atomic war is inevitable. It will destroy half of humanity: it is going to destroy immense human riches. It is very possible. The atomic war is going to provoke a true inferno on Earth. But it will not impede Communism.”

We are preparing ourselves for a stage in which before the atomic war we shall struggle for power, during the atomic war we shall struggle for power and we shall be in power. There is no beginning… there is an end to atomic war, because atomic war is simultaneous revolution in the whole world, not as a chain reaction, simultaneous. Simultaneous doesn't mean the same day and the same hour. Great historic events should not be measured by hours or days, but by periods… The working class will maintain itself, [and] will immediately have to seek its cohesion and centralisation…

After destruction commences, the masses are going to emerge in all countries – in a short time, in a few hours. Capitalism cannot defend itself in an atomic war except by putting itself in caves and attempting to destroy all that it can. The masses, in contrast, are going to come out, will have to come out, because it is the only way to survive, defeating the enemy… The apparatus of capitalism, police, army, will not be able to resist… It will be necessary to organise the workers' power immediately.

Posadas wrote that “Nuclear war [equals] revolutionary war. It will damage humanity but it will not – it cannot – destroy the level of consciousness reached by it… Humanity will pass quickly through a nuclear war into a new human society – Socialism.”

The Posadist group in Cuba, the Revolutionary Workers' Party (Trotskyist) or POR(T), gained importance due to the Cuban Revolution in which it had a minor role. Posadist guerrillas fought alongside Fidel Castro and Che Guevara in 1959. When the Posadists split from the Fourth International in 1962 they took the Cuban section with them leaving no other Trotskyist group represented in Cuba in the 1960s.

In 1961 the POR(T) argued that the Cuban government should forcibly expel the American military base at Guantanamo Bay. According to state media, the Posadists organised workers in the town of Guantanamo to march on the nearby military base. The POR(T) denied the allegation, but it was repeated by the Trotskyist Socialist Workers Party in the US, which was pro-Castro and denounced the Posadists as ultra-left. Cuban officials affiliated with the Popular Socialist Party then raided the headquarters of the Posadist group and smashed its printing press, which was in the process of printing an edition of Trotsky's The Permanent Revolution.

Guevara, when asked in an interview about this event, commented:

"That did happen. It was an error. It was an error committed by a functionary of second rank. They smashed the plates. It should not have been done. However, we consider the Trotskyist party to be acting against the revolution. For example, they were taking the line that the Revolutionary Government is petty bourgeois, and were calling on the proletariat to exert pressure on the government and even to carry out another revolution in which the proletariat would come to power. This was prejudicing the discipline necessary at this stage."

The Cuban Posadist section became increasingly militant and was banned by the government, Castro denonunced them as "pestilential" at the Tricontinental Congress held in January 1966. Cuban Posadists went on to claim that Castro had Guevara killed when, it turned out, he was actually in Bolivia fighting with the guerrilla movement there. Conversely, after Guevara was executed by Bolivian authorities, Posadas claimed in 1967 that it was unclear how Che Guevara was killed.

Beginning in 1968, Posadas also became known for his theories concerning UFOs. If anything like UFOs existed, they could demonstrate the existence of agents able to master a very sophisticated technology, something that would be compatible with what in this planet was advocated by socialism. If UFOs existed, they could be allied in addressing some of the major problems in the earth.

In his pamphlet Les Soucoupes Volantes, le processus de la matiere et de l'energie, la science et le socialisme (Flying Saucers, the process of matter and energy, science and socialism), Posadas pleaded that "“We must call upon beings from other planets when they come to intervene, to collaborate with the inhabitants of the Earth to overcome misery. We must launch a call on them to use their resources to help us.”

Posadas was also thought to have a large ego as indicated by his habit of ending his articles by exclaiming "Long live Posadas!"

This actually stemmed from the way Posadas "wrote" his documents. They were usually the transcript of speeches delivered at party events, whether with a small group of closest collaborators, or larger activities. In the early years, after Posadas ended his speeches with "¡Viva la Revolución Mundial!" and "¡Viva la Cuarta Internacional!", someone else would traditionally yell "¡Viva el camarada Posadas!" – all this would be duly reported in print.